A PIN junction solar cell, in which a P-type semiconductor, an I-type semiconductor, and an N-type semiconductor are joined together, absorbs solar light having energy equal to or higher than the bandgap (Eg) between the conduction band and the valence band of the constituent semiconductors so that electrons are excited from the valence band to the conduction band, and holes are generated in the valence band, and therefore electromotive force is generated in the solar cell.
A PN junction solar cell and a PIN junction solar cell have a single bandgap and are therefore called single-junction solar cells. The PN-junction solar cell and the PIN junction solar cell transmit light having energy lower than the bandgap without absorbing it. On the other hand, energy higher than the bandgap is absorbed, but the absorbed energy higher than the bandgap is consumed as phonons, that is, as thermal energy. Therefore, the single junction solar cells having a single bandgap have a problem that the efficiency of energy conversion is poor.
In order to improve such a problem of energy conversion efficiency, Non-Patent Literature 1 proposes a quantum dot solar cell that utilizes quantum dots having a bandgap of 0.7 to 1.4 eV and quantum dots having a bandgap of 1.4 to 2.1 eV. In the quantum dot solar cell, the quantum dots are regularly arranged to form an intermediate band due to the overlap between the wave functions of the quantum dots, which makes it possible to absorb broadband solar energy.
Further, Non-Patent Literature 2 proposes a novel optical device using a polymer matrix containing semiconductor nanoparticles. This film exhibits the optical quantum size effect of the semiconductor nanoparticles. The quantum dot (QD)/polymer composite film proposed in this literature is prepared by solvent casting of a nanoparticle suspension obtained by suspending a CdSe/ZnS semiconductor in a cellulose triacetate (CTA) solution. Further, Non-Patent Literature 2 states that direct addition of QD to a common CTA casting solution has become possible.